Laser diodes and their associated driver circuits are typically used in the electronic prepress imagesetter industry to convert digital image data, generated by an application running on a host computer system, for example, into images on photosensitive media. The photosensitive media typically contains the arrangement of pixels for a red, green or blue color component of the digitized image.
One common configuration is termed an internal drum imagesetter. In such a device, the photosensitive media is mounted along the inside surface of a drum or cylinder. A laser diode is located at or near the center of the drum on a carriage that allows translation along the drum axis. The output beam from the laser diode is scanned by a rotating mirror across the media in successive circumferentially extending bands or paths referred to as scan lines. The laser diode output beam exposes specific pixel locations of the media along those scan lines to form the desired image. Because the media is associated with a single color component of the image, the laser diode is turned-on or off for those pixel locations that contain that color component and depending on whether a positive or negative image is being generated.
In the past, the laser diode driver circuits have typically been operated at frequencies approaching 1 Megahertz (MHz). Such a laser diode driver circuit is capable of switching the diode on or off within 1 microsecond (i.e. the operating period). Although those laser diode driver circuits can provide the necessary "turn-on current" within that time period, laser diodes typically have a delay of approximately 5 nanoseconds (ns) before beam generation. At that frequency, the 5 ns turn-on time delay represents a relatively inconsequential 0.5 percent of the operating period.
A feedback control circuit typically monitors the beam power generated by the laser diode in relation to a desired beam power. That beam power is typically averaged over one period of the operating frequency by the feedback control circuit. Because the turn-on time delay represents such a small percentage of the overall period, it does not significantly affect the beam power.
Laser diodes, however, have begun to be operated at frequencies exceeding 50 MHz. At such increased frequencies, the operating period is reduced from 1 microsecond to less than 20 nanoseconds. Therefore, the 5 nanosecond turn-on time delay represents at least 25 percent of the total operating period. When the control loop system determines the average beam power output by the laser diode, in relation to the desired power, it erroneously determines that the intensity is 25 percent too low and therefore responsively increases the intensity by 25 percent, for example. Such an intensity increase is undesirable and typically serves to corrupt the resulting image.
A further problem that arises when the operating frequency is increased concerns simulation of the laser diode when it is turned off, such that the feedback control circuit remains in a steady state condition--otherwise, when the laser diode is turned off (for example, in order to impart a black pixel to the photosensitive media) the control loop system will operate in an "open loop" condition. Typically, a dummy laser driver and a dummy load are coupled to the control loop feedback signal when the laser diode is shut off. That dummy laser driver and dummy load are designed to operate in an identical manner to the actual laser diode and laser diode driver circuit. Therefore, when the laser diode is turned back on, the control loop system is already in a steady state condition. Accordingly, little or no compensation to the laser diode output power is required once the laser diode is turned back on.
In the prior art a second laser diode is sometimes used to simulate the dummy load. That approach includes manually inputting calibration factors to adjust the second laser diode such that it more closely simulates the actual laser diode's operation. Such an approach, however, is not automatic and requires time intensive calibration operations.